The present invention relates to a ferroelectric element and, more particularly, to a ferroelectric element which is advantageously usable as a piezoelectric element in ink jet printers and other devices. The ferroelectric element according to the present invention, as compared with the conventional ferroelectric element, can be a thick film having a thickness of 10 to 20 xcexcm and, at the same time, can avoid cracking at the time of the film formation. Further, since fine particles of a ferroelectric precursor can be previously formed in the course of preparation of the ferroelectric element by a sol-gel process, a more dense element can be prepared in a shorter time. Furthermore, the ferroelectric element of the present invention can be advantageously produced from a green sheet thereof. In this case, the firing temperature may be lower than that in the prior art process, and the range of selection of an electrode material to be placed on both of the green sheet necessary for the development of ferroelectricity and piezoelectricity can be broadened. The present invention also relates to a process for producing the above ferroelectric element and a piezoelectric ink jet head using the above ferroelectric element as a piezoelectric element. The term xe2x80x9cpiezoelectricxe2x80x9d and the term xe2x80x9cferroelectricxe2x80x9d used herein are defined as follows. Materials, which, when an external force (a stress from the outside) is applied to a crystal thereof, develop polarization, are called piezoelectrics, and, among the piezoelectrics, those wherein the polarization can be reversed by an external electric field are expressly called ferroelectrics.
In recent years, in office automation (OA) equipment, such as word processors, personal computers, facsimile machines, various measuring instruments, such as medical measuring instruments, and other devices, ink jet printers have been extensively used for printing information from these devices at a high density. As well known in the art, in the ink jet printer, an ink droplet is ejected from a head section of the printer and deposited directly onto a recording medium, such as recording paper, to perform monochrome or color printing. The ink jet printer has many advantages including that printing can be performed on even a three-dimensional recording medium, running cost is low since plain paper can be used as the recording medium, the head can be simply loaded, the need to provide the step of transfer, fixation and the like can be eliminated, color printing is easily performed, and a sharp color printed image can be provided. The head section of the ink jet printer can be classified into several types according to the method for ejecting ink droplets from the head section. Among them, a typically and advantageously used one is a piezoelectric ink jet head.
The piezoelectric ink jet head generally comprises: a plurality of ink chambers which are disposed at equidistant spaces and function as an ink flow passage and a pressurizing chamber for ejecting an ink; and a nozzle plate mounted on the front end of the ink chambers and equipped with nozzles, for ejecting an ink, corresponding respectively to the ink chambers; and pressurizing means for pressurizing an ink within the ink chamber in response to a demand for printing. The pressurizing means comprises a piezoelectric element, and an electrostrictive effect attained by this piezoelectric element is utilized to create a pressure wave within the ink chamber, filled with an ink, in the head section, permitting the ink to be ejected through the nozzle in the head section.
Ferroelectric elements have been extensively used as a piezoelectric element in the above ink jet head or as, for example, capacitors, actuators, memories, and other elements. A ferroelectric element consists essentially of a ferroelectric body or a ferroelectric material. Examples of typical ferroelectric materials include lead zirconate titanate (PZT) generally represented by Pb(Zr,Ti)O3, (Pb,La)(Zr,Ti)O3 (PLZT), and Pb(Mg1/3Nb2/3)O3 (PMN). In particular, it is known that ferroelectrics containing lead (Pb) as one metal component, including PZT, have large remanence, specific permittivity, and piezoelectric constant and possess excellent piezoelectricity and ferroelectricity. In the present specification, the ferroelectric material will be described particularly with reference to PZT.
The above ferroelectric elements, particularly thin film elements of PZT, have hitherto been produced by various film forming methods, such as sputtering, sol-gel process, CVD, and laser ablation, or methods related thereto. When the thin film element is formed particularly in a large thickness of 10 to 20 xcexcm, a method has been used wherein the film thickness is increased by prolonging the film formation time or by repeating the film formation procedure. Further, when a PZT element having a perovskite structure is produced, firing is generally performed in a high temperature atmosphere of 500 to 800xc2x0 C.
Among the film formation methods, the sol-gel process which is particularly included in the range of a solution preparation method is advantageous in that a high-purity thin film of PZT can be formed, a starting material can be quantitatively dissolved in a solution and, hence, the composition of the formed thin film of PZT can reflect the composition of the starting material used, which facilitates the control of the composition and can provide a thin film of PZT having high surface smoothness by repetition of spin coating and firing. The solvent used in the preparation of a sol-gel solution is in many cases an alcohol solvent because the metal for PZT takes the form of a metal alkoxide or a metal salt of an organic carboxylic acid. The solution prepared by the sol-gel process may be coated onto a substrate, for example, by spin coating or dip coating to form a film. In this film formation, addition of a photosensitive resin to the solution enables patterning by photoetching.
More specifically, for example, Japanese Unexamined Patent Publication (Kokai) No. 6-112550 discloses a method for forming a thin film of PZT which comprises hydrolyzing a metal alkoxide as a PZT material to prepare a sol solution, adding a soluble organic polymer, for example, polyethylene glycol monomethyl ether, to the solution and thoroughly stirring the solution. Subsequently, a platinum electrode is formed on a silicon substrate, a sol solution prepared above is spin-coated on the electrode, and the coating is heated to about 350xc2x0 C. The prefiring results in the formation of a porous thin film of a gel. The same starting material as the above PZT material is hydrolyzed to form a sol solution. In this case, however, no polyethylene glycol monomethyl ether is added. The sol solution is spin-coated onto the above porous thin film of a gel to form a coating which is then dried by heating at 400xc2x0 C. The resultant thin film is fired in an oxygen atmosphere for 15 hr. The firing temperature is generally 600 to 700xc2x0 C. Thus, a thin film of PZT having a perovskite structure can be formed through a series of steps. In this film formation method, a sol is filled into pores of the porous thin film of a gel. This reduces the porosity and can offer high Young""s modulus and consequently excellent electric properties. Further, since the size of the pore is not more than 1 xcexcm, no cracks are created.
Japanese Unexamined Patent Publication (Kokai) No. 6-119811 discloses a process for producing a ferroelectric thin film element, comprising producing a ferroelectric thin film by a sol-gel process using a metal alkoxide as a main starting material, wherein particles of a ferroelectric oxide are added to a sol prepared by hydrolyzing the starting material followed by homogeneous mixing to prepare a coating liquid. In this film formation method, addition of ferroelectric oxide particles to the sol enables a thick film to be easily formed and, in addition, results in the formation of a thin film of PZT having excellent properties and electric characteristics.
When all the above prior art methods are taken into consideration, it can be said that, in the conventional methods for producing a ferroelectric element, it is very difficult to produce a ferroelectric thin film element having a relatively large thickness of 1 xcexcm or more. Further, according to a sol-gel process which can generally provide a film of a submicron thickness, repetition of coating of a coating solution prepared by the sol-gel process can provide a thick film. In this case, however, it is difficult to form a dense film having excellent qualities.
Further, the formation of the thick film poses a problem that cracking is often created to make it impossible to provide ferroelectric properties. Specifically, for example, coating of an aqueous PZT precursor solution by a conventional method, such as dip coating or spin coating, followed by drying, degreasing and firing to form a thin film of PZT having a thickness of not less than 1 xcexcm often results in the creation of cracking. The creation of cracking could not be avoided even when the thin film of PZT is formed by stacking a plurality of thinner films on top of another. Creation of cracking in the thin film of PZT results in lowered film density, makes it impossible to form an element, such as an electrode, on the top surface of the film, and, hence, makes it impossible to utilize the thin film of PZT, for example, as a piezoelectric element of an ink jet head. In fact, since the film thickness of the piezoelectric element used in the head of the ink jet printer is generally about 10 to 20 xcexcm, the conventional methods are unsatisfactory also from the practical viewpoint.
Referring again to Japanese Unexamined Publication (Kokai) No. 6-119811, as described above, it teaches that, in the preparation of a thin film of PZT by the sol-gel process, after the preparation of a PZT sol-gel solution, separately prepared fine particles of PZT are mixed with the PZT sol-gel solution to prepare a homogeneous coating liquid. This method, however, is troublesome because the PZT sol-gel solution and the fine particles of PZT to be added to the PZT sol-gel solution should be prepared separately. Further, a PZT sol-gel solution is prepared by the method which is different from the method by which the fine particles of PZT are prepared. Therefore, in the addition of the fine particles to the sol-gel solution, it is difficult to achieve homogeneous dispersion.
Preparation of a ferroelectric element through a green sheet of a precursor to the ferroelectric element has also been extensively carried out in the art. In this preparation method, in general, a fine powder of a ferroelectric material is mixed with a suitable binder material and a solvent, and the resultant mixed liquid is formed into a sheet by using a coating device, such as a doctor blade or a roll coater, or screen printing. The green sheet is fired at 1000xc2x0 C. or above, and a conductive paste is coated thereon, followed by firing at the firing temperature of the conductive paste. In this method, however, the adoption of a high temperature of 1000xc2x0 C. or above as the firing temperature poses a problem that elements constituting the dielectric are unfavorably evaporated during firing, leading to a variation in composition ratio in the resultant ferroelectric and, hence, rendering the control of the composition ratio difficult. The high-temperature firing poses an additional problem that the electrode material to be coated onto a green sheet for developing the ferroelectricity and the piezoelectricity is limited to those having heat resistance.
An object of the present invention is to eliminate the above many problems in the prior art.
It is therefore an object of the present invention to provide a ferroelectric element wherein the film thickness can be easily increased by the sol-gel process without creating any cracks in the film and the film has quality sufficient for practical use, and, at the same time, is dense.
It is another object of the present invention to provide a ferroelectric element, produced by a process involving the step of adding fine particles of a ferroelectric precursor to a sol-gel solution of the ferroelectric precursor, wherein the fine particles of a ferroelectric precursor can be produced in situ in a sol-gel solution in the course of preparing the sol-gel solution and, hence, the need to specially prepare fine particles of the ferroelectric precursor in other sites can be eliminated.
It is still another object of the present invention to provide a ferroelectric element which, as a result of the in-situ production of fine particles of a ferroelectric precursor in sol-gel solution, can omit the time necessary for the intentional preparation of fine particles, permits fine particles to be homogeneously dispersed in a sol-gel solution and, therefore, can be provided in a denser thin film form.
It is a further object of the present invention to provide a ferroelectric element which, particularly when a ferroelectric element is prepared through a green sheet of the precursor, permits the composition ratio in the ferroelectric to be easily controlled, enables the firing temperature to be lowered, and enables the range of selection of an electrode material for providing ferroelectricity and piezoelectricity to be broadened.
In addition to the above objects, an additional object of the present invention is to provide a process for producing the above excellent ferroelectric element.
Another object of the present invention is to provide a piezoelectric ink jet head using the ferroelectric element of the present invention as a piezoelectric element.
The above and other objects of the present invention could be easily understood from the following detailed description.
According to one aspect of the present invention, there is provided a ferroelectric element, comprising a ferroelectric material containing at least two metals, said ferroelectric element having been produced by a process comprising the steps of: providing alkoxides of the respective metals as starting compounds; hydrolyzing the metal alkoxides in the presence of a catalytic amount of a protonic acid to increase the molecular weight; and adding a film-forming polymer material to the sol-like solution prepared in the step of increasing the molecular weight to prepare a sol-like ferroelectric precursor solution.
In the ferroelectric element according to the present invention, preferably, the sol-like ferroelectric precursor solution further comprises fine particles of a ferroelectric precursor produced in situ in the course of the preparation of the sol-like ferroelectric precursor solution. In this case, preferably, the fine particles of the ferroelectric precursor have been produced by adding a binder material, to the ferroelectric precursor solution, in an amount effective in accelerating the gelation.
The protonic acid, which is allowed to exist as a catalyst in the hydrolysis of the metal alkoxide, is preferably hydrochloric acid, sulfuric acid, nitric acid or the like, and these protonic acids may be used alone or in a combination of two or more.
Preferred examples of the film-forming polymer material, which is added for the preparation of a sol-like ferroelectric precursor solution, include alkyl celluloses, hydroxyalkyl celluloses, polyethylene oxide, polyvinyl alcohol, polyvinyl butyral, polyvinyl pyrrolidone, and polyacrylic esters, and they may be used alone or in a combination of two or more.
The binder material added for the preparation of fine particles of a ferroelectric precursor may be the same as or different from the film-forming polymer material added in the previous step of preparing a sol-like ferroelectric precursor solution. Preferably, however, the binder material is the same as the film-forming polymer material, more preferably a vinyl polymer material, particularly preferably polyvinyl butyral.
The ferroelectric material for constituting the ferroelectric element of the present invention is not particularly limited so far as the conditions specified in the present invention are satisfied. Preferred examples thereof include ceramics having a perovskite structure with lead zirconate titanate (PZT), PLZT, PMN and the like being more preferred. Use of PZT is particularly advantageous for carrying out the present invention.
The ferroelectric element of the present invention may be provided in various forms. In general, however, it is preferably in the form of a thin film having a thickness of 10 to 20 xcexcm.
According to another aspect of the present invention, there is provided a process for producing a ferroelectric element comprising a ferroelectric material containing at least two metals, comprising the steps of:
hydrolyzing alkoxides of the respective metals in the presence of a catalytic amount of a protonic acid to increase the molecular weight of the metal alkoxides;
adding a film-forming polymer material to the resultant sol-like solution; and
treating the resultant sol-like ferroelectric precursor solution to prepare a desired ferroelectric element.
According to one preferred embodiment of the present invention, the process for producing a ferroelectric element further comprises, before the step of treating the sol-like ferroelectric precursor solution prepared in the step of increasing the molecular weight of the metal alkoxides, the step of adding a binder material, to the ferroelectric precursor solution, in an amount effective in accelerating the gelation to produce fine particles of the ferroelectric precursor in the solution.
As described above, the ferroelectric element produced according to the process of the present invention is preferably in a thin film form, and, therefore, preferably, the step of coating the sol-like ferroelectric precursor solution on a substrate and drying the coating is repeated to a predetermined coating thickness and the coating is finally fired to form a thin-film ferroelectric element. If necessary, the step of coating the sol-like ferroelectric precursor solution on a substrate and drying and firing the coating is repeated to form a thin-film ferroelectric element having increased thickness.
In the addition of the binder material to the sol-like ferroelectric precursor solution according to the present invention, the binder material is preferably added in an amount large enough to permit the sol-gel reaction to further proceed to produce fine particles of a ferroelectric precursor in the resultant sol-gel solution. The sol-gel solution may be then formed into a green sheet having a predetermined shape which is finally annealed to produce the ferroelectric element of the present invention.
According to another aspect of the present invention, there is provided an ink jet head comprising: a plurality of nozzles for ejecting an ink; an ink chamber, communicating with the nozzles, for permitting therein passage and pressurization of the ink; and pressurizing means for creating a change in volume of the ink in the ink chamber to eject the ink through the nozzle, wherein the pressurizing means comprises, as a piezoelectric element, a ferroelectric element comprising a ferroelectric material containing at least two metals, said ferroelectric element having been produced by a process comprising the steps of: providing alkoxides of the respective metals as starting compounds; hydrolyzing the metal alkoxides in the presence of a catalytic amount of a protonic acid to increase the molecular weight of the metal alkoxides; and adding a film-forming polymer material to the sol-like solution prepared in the step of increasing the molecular weight to prepare a sol-like ferroelectric precursor solution.
Basically, the ink jet head of the present invention has the same construction as the piezoelectric ink jet head commonly used in the art and is not particularly limited so far as the described thin film of a ferroelectric is used as a piezoelectric element. Therefore, a suitable ink jet head is, for example, one which will be described below with reference to FIG. 5.